Molten or liquid-phase metal, particularly molten aluminum, in practice generally contains entrained solids which are deleterious to the final cast metal product. These entrained solids appear as inclusions in the final cast product after the molten metal is solidified and cause the final product to be less ductile or to have poor bright finishing and anodizing characteristics. The inclusions may originate from several sources. For example, the inclusions may originate from surface oxide films which become broken up and are entrained in the resultant molten metal. In addition, inclusions may originate as insoluble impurities, such as carbides, borides and others, or eroded furnace and trough refractories.
The filtering of molten metal to remove impurities is routinely done before casting, especially in the casting of aluminum. Heretofore, the filtering is accomplished by pouring the molten metal into a vessel which has a filter on or near its lower surface and allowing the force of gravity to cause the molten metal to flow through the filter. These processes suffer from one or more serious disadvantages making them less than entirely optimal for use with molten metals.
Among the disadvantages of existing processes are that they can be slow since they are dependent on the force of gravity to pull the liquid-phase metal through the filtering means. Often the option to use many filters is not available because of the low permeability of the filtration media.
In addition, prior processes which depend on gravity to pull the liquid-phase metal through the filtering means clog quickly because the change in pressure available to force the metal through is low and cannot be adjusted as the filter starts to plug.
Furthermore, many prior processes which are not dependent on gravity to generate the pressure differential to force the liquid-phase metal through the filtering means require the use of equipment with moving parts, bearings and seals which eventually need to be replaced. Downtime due to maintenance of this equipment adds to the overall cost of production.
It is known that liquid-phase metals can be circulated by electromagnetic pumps. Electromagnetic pumps are pumps which operate on the principle that a force is exerted on a conductor (the liquid) carrying current in a magnetic field. The high electrical conductivity of liquid metals makes it possible to pump them by magnetic means. Various electromagnetic pumps have been found to be useful in the heating, mixing and casting of metals.
Electromagnetic pumps may be either a direct-current(dc) conduction pump or an alternating-current(ac) induction pump. Direct-current conduction pumps are a direct application of the right-hand rule, which states that a current passing at right angles to a magnetic field will produce a force at right angles to both. This force, directed in the fluid, manifests itself as a pressure if the fluid is suitably contained. Pump performance of direct-current electromagnetic pumps depends upon the magnitude of the current, magnetic field intensity, and the geometry of the pump duct. The disadvantage of this type of pump is that very high current (thousands of amperes) at low voltage (1-2 volts) is required.
Large currents can also be developed in liquid metal by electromagnetic induction. Alternating-current induction pumps consist of a duct in the form of a flattened tube extending between two core sections containing a three-phase ac winding. The winding is similar to that of an induction motor stator except that the field structure is flat and a sliding rather than a rotating magnetic field is produced. The field windings must be cooled to protect the electrical insulation. Alternatingcurrent pumps typically employ conventional power supplies (60-hertz ac) but variable power supplies may be used.
Examples of various electromagnetic pumps are disclosed in the following U.S. Pat. Nos. 2,786,416; 2,929,326; 4,212,592 and the references cited therein.
A principal object of the present invention is to provide a method and apparatus for filtering liquid-phase metal which does not suffer from the disadvantages and limitations of prior filtering methods and apparatus.
Another object of the present invention is to provide a method and apparatus for filtering liquid-phase metal which does not rely solely on the force of gravity to pull the liquid-phase metal through the filtering means.
Another object of the present invention is to provide a method and apparatus for filtering liquid-phase metal which will not clog quickly because the change in pressure available to force the metal through can be adjusted as the filter starts to plug.
Still another object of the present invention is to provide an apparatus for filtering liquid-phase metal that is capable of utilizing filters having a lower permeability than filters which have been used heretofore.
Yet another object of the present invention is to provide a method and apparatus for filtering liquid-phase metal which does not require pouring the metal and thereby minimizes the amount of liquid-phase metal which contacts the atmosphere.
A further object of the present invention is to provide a method and apparatus for filtering liquid-phase metal which minimizes production downtime due to maintenance of moving parts, bearings or seals which are required to transfer the liquid-phase metal to the filter.
A further object is to provide a means of varying pressure so that as the filter clogs, more pressure is applied in a controlled fashion to assure metal flow through the filter.
Additional objects and advantages of the invention will be more fully understood and appreciated with reference to the following description.